Vacuum pumps

ABSTRACT

A vacuum pump includes a pump body in which is mounted two externally threaded rotors. The thread profile of each rotor is tapered in a substantially radial direction to facilitate the machining and fitting of the rotors with the required close tolerances.

FIELD OF THE INVENTION

[0001] The present invention relates to vacuum pumps of the screw typeand, more particularly, to such pumps with an improved screw threaddesign.

BACKGROUND OF THE INVENTION

[0002] Vacuum pumps having a screw mechanism and comprising twoexternally threaded rotors mounted in the pumping chamber of a pump body(stator) and adapted for counter-rotation in the chamber withintermeshing of the rotor threads are known. Close tolerances betweenthe rotor threads at the points of intermeshing and with the pumpingchamber surfaces causes volumes of gas being pumped between an inlet andan outlet to be trapped between the threads of the rotors and saidsurfaces and thereby urged through the pump as the rotors are rotating.

[0003] Such pumps are potentially attractive because they can bemanufactured with few working components and have the ability to pumpfrom a high vacuum environment at the inlet down to atmospheric pressureat the outlet.

[0004] However certain disadvantages are evident from existing designsof screw vacuum pump, in particular that they generally require a highpower consumption in view of the fact, for example, that the action ofthe intermeshing rotors does not provide for compression of the gas asit passes through the pump.

[0005] A further disadvantage is that the thread profiles of each rotormust be machined and fitted to close tolerances to allow the threads ofthe rotors to intermesh correctly with close tolerance but withoutcontact between the respective threads.

[0006] It is an aim of the present invention to provide a vacuum pumpincorporating a screw mechanism the threads of which are profiled tofacilitate the machining and fitting of the rotors whilst ensuring thatno direct contact between the rotors and between each rotor and the pumpbody (stator) takes place.

SUMMARY OF THE INVENTION

[0007] According to the present invention there is provided a vacuumpump incorporating a screw mechanism and comprising two externallythreaded rotors mounted in a pump body and adapted for counter-rotationin the body with intermeshing of the rotor threads and with closetolerances between the threads and internal pump body surfaces in orderto pump gas from a pump inlet to a pump outlet by action of the rotorsand wherein the thread profile of each rotor is tapered in asubstantially radial direction.

[0008] Advantageously, the tapering is effected from close to the pitchcircle diameter of each rotor. The tapering angle in general mayusefully be of the order of 1° to 6° on each side of the thread,preferably 2° to 3°, for example 2.5°.

[0009] The inlet to the pump may be substantially at the centre of thepump so that gas being pumped is directed directly in to the gap formedbetween the top of the rotors and to stator body.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] An embodiment of the invention will now be described by way ofexample, reference being made to the Figures of the accompanyingdiagrammatic drawings in which:

[0011]FIG. 1 is a view of the pumping chamber of a vacuum pump accordingto the present invention;

[0012]FIG. 2 is an enlarged view showing the profiles of twointermeshing threads at the centreline of the pumping chamber; and

[0013]FIG. 3 is a sectional profile view (not to scale) of the pumpingchamber of the vacuum pump of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] As shown, a vacuum pump comprises a pump body 1 which togetherwith a top plate 2 and a bottom plate 4 defines a pumping chamber 20.The top plate 2 has formed therein an inlet 3 to the pumping chamber 20and the bottom plate 4 has formed therein an outlet 5 from the pumpingchamber to atmosphere.

[0015] As illustrated most clearly in FIG. 3, the pumping chamber 20 isin the form of a ‘figure of eight’, the substantially circularcross-sectional portions of which taper gradually in the direction fromthe inlet 3 towards the outlet 4.

[0016] Received in the circular cross-sectional portions of the pumpingchamber 20 are two rotors 7,8 of cylindrical type shape overall but eachcomprising a root portion 9, 10 respectively, the (root) diameter D₁ ofwhich increases gradually in a direction from the pump inlet 3 to thepump outlet 5, on which is present a continuous helical thread, the(thread) diameter D₂ of which decreases gradually in a direction fromthe pump inlet 3 to the pump outlet 5.

[0017] The rotors 7, 8 are attached to respective shafts 11, 12 whichare adapted for rotation in opposite directions (counter-rotation)within bearings 13, 14 respectively at the top of the shafts 11, 12 andbearings 15, 16 respectively at the bottom of the shafts 11, 12.

[0018] In use, counter-rotation of the shafts is effected by a motorpositioned about the shaft 11 with a connection with the shaft 12 bymeans of interconnecting gears attached to the respective shafts suchthat, in use, both shafts rotate at the same speed but in oppositedirections.

[0019] The overall shape of the pumping chamber 20 and the rotors, 7, 8and in particular the external thread diameter of the rotors 7, 8 areall carefully calculated to ensure close tolerances between the outerthread surfaces and the pumping chamber 20 and to ensure closetolerances also in the vicinity of the centreline of the pump where thethreads of the rotors 7, 8 intermesh. All tolerances must ensure nodirect contact between the rotors and between each rotor and the pumpbody (stator) but effecting as small a leakage path between the parts aspossible.

[0020] According to the present invention, this is effected as shownmost clearly in FIG. 2 by having thread profile of each rotor taper in aradial direction from close to the pitch circle diameter D₃ by an angleA as shown in FIG. 2 on each side of the helical thread. A preferredangle A of 2.5° is shown in FIG. 2 but an angle of taper between 1° and6° on each side of the thread is effective.

[0021] In a preferred embodiment the taper is started from just insidethe pitch circle.

[0022] Such tapering allows in general for a ready accommodation of thetapered rotors in pumps of the invention with the required closetolerances.

I claim:
 1. A vacuum pump incorporating a screw mechanism and comprisingtwo externally threaded rotors mounted in a pump body and adapted forcounter-rotation in the body with intermeshing of the rotor threads andwith close tolerances between the threads and internal pump bodysurfaces in order to pump gas from a pump inlet to a pump outlet byaction of the rotors and wherein the thread profile of each rotor istapered in a substantially radial direction.
 2. The vacuum pump asclaimed in claim 1 , in which the tapering is effected from close to thepitch circle diameter of each rotor.
 3. A vacuum pump as claimed inclaim 1 , in which the angle of taper is between 1° and 6° on each sideof the thread.
 4. The vacuum pump as claimed in claim 3 , in which theangle of the taper is between 2° to 3° on each side of the thread. 5.The vacuum pump as claimed in claim 3 , in which the angle of taper is2.5°.
 6. The vacuum pump as claimed in claim 2 , in which the taperingis effected from inside the pitch circle diameter of each rotor.